This invention relates generally to the field of drug infusion systems. More specifically, it relates to a method and apparatus for compressing or collapsing a compliant or flexible container or reservoir for a liquid drug to cause the contents of the container or reservoir to flow through a delivery tube like under substantially constant pressure.
Many drugs or pharmacologically active medicaments are introduced into a patient's bloodstream or bodily tissues by drug delivery systems that produce a controlled flow of the liquid into the patient's body. Among such systems are intravenous (IV) systems in which the liquid is stored in a bag external to the body and is then conducted into the body (either by an external pump or gravity) through a flexible conduit and an IV needle; and implantable systems, in which a liquid reservoir, a pump, and a delivery tube are implanted in the body. For the purposes of this disclosure, both types of drug delivery systems may be termed "infusion systems".
It is typical in such systems to require a controlled rate of delivery of the liquid. In gravity flow IV systems, for example, a valving mechanism may provide such a controllable flow rate. Other infusion systems employ an infusion pump for this purpose. The infusion pump may be powered by various means. For example, pumps employing a solenoid-actuated mechanism are disclosed in the following U.S. Patents: U.S. Pat. Nos. 4,360,019--Portner et al.; 4,557,726--Reinicke; and 4,594,058--Fischell. Pumps employing a spring-actuated mechanism are disclosed in U.S. Pat. Nos. 4,447,232--Sealfon et al. and 5,328,477--Sitko. Electric motor-driven pumps are disclosed in U.S. Pat. Nos. 5,23,439--Campbell et al. and 5,330,431--Herskowitz. A pump that employs a gas-operated mechanism is disclosed in U.S. Pat. No. 5,242,406--Gross et al. A combination of electromagnetic and spring-actuated means for an infusion pump is disclosed in U.S. Pat. No. 4,274,407--Scarlett.
The typical prior art infusion pump, as exemplified by the above-mentioned references, requires a relatively bulky and/or complex mechanism to provide the desired control over the flow rate. One factor that contributes to the complexity of those infusion pumps that apply a compressive force to a bag-like or bladder-like deformable (compressible) reservoir is the changing geometry of the reservoir as it empties. Specifically, where the pumping mechanism applies a compressive force to the compressible reservoir, the changing geometry of the reservoir results in a change in the area against which the force is applied, thereby changing the applied pressure (force per unit area). Typically, as the reservoir's volume is reduced, the surface area on which the compressive force is applied increases, thereby resulting in a decrease in force per unit area (pressure). Therefore, where it is desired to maintain a substantially constant pressure, or an increasing pressure, for the fluid flow from the reservoir, the pumping mechanism must include means for compensating for the effect of increasing reservoir surface area exposed to the compressive force.
Several of the above-mentioned patents disclose devices that mechanically compress a deformable reservoir bag. See U.S. Pat. Nos. 4,274,407--Scarlett; 4,447,232--Sealfon et al.; 5,232,439--Campbell et al.; 5,242,406--Gross et al.; 5,328,477--Sitko; and 5,330,431--Herskowitz. Of these, U.S. Pat. No. 5,232,439--Campbell et al. most clearly recognizes the problem of compensating for increasing reservoir surface area exposed to the compressive force. The device disclosed in this patent employs motor drive controls to control the movement of a pressure plate against the reservoir bag. U.S. Pat. No. 5,330,431--Herskowitz also recognizes the problem of maintaining constant pressure to the reservoir bag. Like the device in the Campbell et al. patent, the Herskowitz device uses motor drive controls to control the movement of the bag compressing element.
While the prior art has thus addressed the problem of maintaining a substantially constant pressure applied to a deformable reservoir bag in an infusion pump, it has done so with devices that include relatively complex and costly control mechanisms. It would be a marked advance over the prior art to provide an infusion pump that satisfactorily addresses this problem with a relatively simple mechanism that may be manufactured economically, and that is simple to use and easy to maintain in working order. Furthermore, it would be advantageous for such an infusion pump to require no electrical power, so as to avoid the need for changing batteries. In addition, it would be a still further advantage for such an improved pump to accommodate standard pre-filled reservoir bags.